1. Field of the Invention
This invention relates to a voltage detecting device for detecting a voltage developed at a predetermined part of an object under measurement such as an integrated circuit, and more particularly to a voltage detecting device that utilizes the principle that the polarization of a light beam is changed by a voltage developed at a predetermined part of an object under test.
2. Prior Art
A variety of voltage detecting devices have been employed for detecting a voltage developed at a predetermined part of an object under measurement such as an integrated circuit. One example of a voltage detecting device of this type, which has been developed recently, detects the voltage of an object under test by utilizing the principle that the polarization of a light beam is changed by a voltage provided at a predetermined part of an object under measurement. A voltage detecting device of this type is disclosed in Japanese Unexamined Patent Application No. 137317/87 filed on May 30, 1987.
An optical probe having an extremely small sectional area includes an electro-optic material that has a refractive index that is affected by the voltage of an object under test. A light beam with a predetermined polarization component is applied to the electro-optic material, and variations in polarization of the light beam caused by the change in refractive index of the electro-optic material is detected for measurement of a voltage developed at one part of the object under test. Such a device is shown in FIG. 1.
The voltage detecting device 50, as shown in FIG. 1, comprises an optical probe 52, a light source 53 for example, a laser diode, an optical fiber 51 for leading the output light beam of the light source 53 through a condenser lens 60 to the optical probe 52, and a detector 55 to which a reference light beam REF and an emergent light beam SG from the optical probe 52 are applied.
The optical probe 52 is formed from an electro-optic material 62 of an optically uniaxial crystal such as lithium tantalate (LiTaO.sub.3), and has an end portion 63 tapered like a truncated cone. A conductive electrode 64 is formed on the outer cylindrical wall of the optical probe 52. A reflecting mirror 65 of dielectric multi-layer film or metal film is bonded to the top of the end portion 63.
The optical probe 52 further includes a collimator 94, condenser lenses 95 and 96, a polarizer 54 for extracting a light beam having a predetermined polarization component out of the output light beams of the collimator 94, and a beam splitter 56 that divides the output light beam of the polarizer 54 into the reference light beam REF and an incident light beam IN and applies an emergent light beam from the electro-optic material 62 to an analyzer 57. The reference light beam REF and the output light beam SG are applied through the condenser lenses 95 and 96 and the optical fibers 58 and 59, respectively, to the detector 55.
In operation, the conductive electrode 64 of the optical probe 52 is held at ground potential. Under this condition, the end portion 63 of the optical probe 52 is placed near an object under test, for instance an integrated circuit (not shown). As a result, the refractive index of the end portion 63 of the electro-optic material 62 in the optical probe 52 is changed. More specifically, in the optically uniaxial crystal, the difference between the refractive index of an ordinary light beam and that of an extraordinary light beam in a plane perpendicular to the light-traveling direction is changed.
The output light beam of the light source 53 is applied through the condenser lens 60 and the optical fiber 51 to the collimator 94. The output light beam of the collimator 94 is applied to the polarizer 54, where it is converted into a light beam having a predetermined polarization component and an intensity of I. The output light beam of the polarizer 54 is applied through the beam splitter 56 to the electro-optic material 62 in the optical probe 52. Each of the reference light and the input light provided by the beam splitter 56 has an intensity of I/2.
As described above, the refractive index of the end portion 63 of the electro-optic material 62 is affected by the voltage of the object. Therefore, the incident light beam IN applied to the electro-optic material 62 is changed in polarization with the refractive index of the end portion 63 of the electro-optic material 62, and reflected by the reflecting mirror 65. The reflected light beam is allowed to advance, as an emergent light beam, to the beam splitter 56. The polarization of the incident light beam IN changes in proportion to the difference in refractive index between the ordinary light beam and the extraordinary light beam (a light beam passing through electro optic material wherein the polarization has been changed due to the voltage of the test object) which is caused by the voltage of the test object and a value 2l (where l is the length of the end portion 63 of the electro-optic material 62).
The emergent light beam is applied to the analyzer 57 by the beam splitter 56. The intensity of the emergent light beam applied to the analyzer 57 is reduced to I/4 by the beam splitter 56. In the case where the analyzer 57 is designed to transmit only a light beam having a polarization component perpendicular to the polarization component of the polarizer 54, the intensity I/4 of the emergent light beam applied to the analyzer 57 is converted into (I/4) sin.sup.2 [(.pi./2).multidot.V/V.sub.0)] (where V is the voltage of the object under test, and V.sub.0 is the half-wave voltage by the analyzer.
The intensity (I/4).multidot.sin.sup.2 [(.pi./2)V/V.sub.0)] of the emergent light beam SG changes with the refractive index of the end portion 63 of the electro-optic material 62 which changes with the voltage of the object. Therefore, the detector 55 can detect the voltage provided at the predetermined part of the test object, such as an integrated circuit.
As described above, with the voltage detecting device 50 shown in FIG. 1, a voltage provided at a predetermined part of an object under test is detected from the change in refractive index of the end portion 63 of the electro-optic material 62 caused by placing the end portion 63 near the predetermined part of the object. Therefore, in the case where it is difficult to bring the optical probe into contact with a small part of an object under test, such as an integrated circuit, or where touching the test object with the optical probe may adversely affect the detection of the voltage, the voltage can be positively detected with the optical probe set apart from the object.
If a pulse light source such as a laser diode outputting a pulse light beam having an extremely small pulse width is employed as the light source 53 to detect high-speed voltage changes in the object under test when sampled at extremely short time intervals, or if a CW (Continuous-Wave) light source is employed as the light source 53 while a high-speed response detector such as a streak camera is used as the detector 55 so that high-speed voltage changes of the object may be measured with high time resolution, then high-speed voltage changes can be detected with high accuracy.
In the voltage detecting device 50 shown in FIG. 1, the optical probe 52 is extremely small in cross section, and the device 50 is provided for measurement of a voltage provided at only one point (position) of an object. Accordingly, when it is required to measure voltages at a plurality of points of an object, the operator must manually move the optical probe 52 to each of the points on the object. This manual movement of the optical probe is rather troublesome and time consuming.
Thus, the conventional voltage detecting device cannot be used to detect the voltages at a plurality of locations or parts of an object under test simultaneously. Also, it is physically difficult to miniaturize the optical probe, and it is difficult to improve the spatial resolution and thereby improve voltage measurement accuracy.